1. Field of the Invention
The present invention relates to a semiconductor substrate having an SOI structure (Si/SiO.sub.2 structure) formed by bonding two wafers integrally, and more particularly to a semiconductor substrate having an SOI structure formed by bonding a silicon wafer and a quartz wafer together and a method of producing said semiconductor substrate having an SOI structure.
2. Prior Art
Semiconductor substrate having an SOI structure formed by bonding two wafers integrally are quite advantageous as base substrates from which highly densified semiconductor integrated circuit devices can be constituted and in recent years they are, for example, used widely as structural substrate for forming liquid crystal devices for HD televisions such as TFT transistors and their peripheral drives and control circuits.
Japanese Patent Application Publication No. 13155/1975 suggests, for example, as a means of producing such semiconductor substrate having an SOI structure, a technique for bonding two wafers firmly together by heating the two wafers at a high temperature (for example about 1,200.degree. C.) under an oxidizing atmosphere with them firmly in contact with each other.
However, in the case wherein a silicon wafer and a quartz wafer are bonded, according to experiments conducted by the present inventors, if the surfaces to be bonded are polished and cleaned, the wafers can be bonded relatively strongly even at a room temperature, but when the surface of the silicon wafer is subjected to mechanical surface grinding by a surface grinder which is a step before the polishing step by abrading, the bonding power between the bonded wafers is still insufficient and further, since the thermal expansion coefficients of them are different (thermal expansion coefficient of silicon: 2.33.times.10.sup.-6 ; thermal expansion coefficient of quartz glass: 0.6.times.10.sup.-6), thermal strain occurs in one of the wafers during heat treatment for temporarily bonding the wafers or during cooling treatment after the full bonding, leading to a risk that a crack, partial separation or the like will develop to damage the wafers.
To obviate such disadvantages, although there are suggested methods wherein two wafers are not stuck as above but an amorphous silicon (a-Si) film is directly formed on a glass substrate corresponding to a quartz wafer by vapor deposition or other means or a polycrystalline silicon film is formed on a glass substrate, for example, by CVD, in the a-Si film and polycrystalline silicon film, the mobility of electrons in the substrate is greatly restricted and therefore the substrate is not satisfactorily suitable for HD televisions and extra-high-frequency and extra-high luminance display devices which will be developed in the future.
That is, when a technique for forming layers by the formation of the above a-Si film is applied for the formation of TFT-LCD (thin-film transistor liquid crystal display) or the like, even though TFT transistors are embedded highly densely, there is a limit to the density of pins to be connected to external drive circuits and therefore to effect the packaging is quite difficult.
On the other hand, when the latter polycrystalline silicon wafer is used instead of the wafer wherein an a.Si film is formed on a glass substrate, the mobility of electrons can be increased in comparison with the case of the a-Si film and since driving circuits can be integrally formed around the TFT region by bonding the polycrystalline silicon wafer onto the quartz glass plate (wafer), it appears that there is no problem of the packaging. However, in the structural substrate for forming a TFT-LCD, it is required to bring the thickness of the silicon layer to an order of 1 .mu.m and it is required that the bonding power between the quartz wafer and the silicon thin film thereon is sufficient because they will be subjected to a thermal or mechanical impact force in the process of the formation of electronic devices, but, at present, the above-mentioned technical problem involved in the difference of thermal expansion coefficient is remained unsolved.